Organic light-emitting devices (OLEDs), for example organic light emitting diodes, are broadly researched and utilized for their application in flat-panel displays. Flat-panel displays employing organic light-emitting devices are brighter than liquid crystal displays (LCDs) because organic light-emitting devices can emit light themselves and do not require backlight systems. Additionally, with different organic materials, organic light-emitting devices can emit light in red, green and blue colors with high luminescence efficiency. Moreover, organic light-emitting devices can operate with low driving voltages and are viewable from oblique angles.
Organic light-emitting devices are usually structured to have a plurality of layers, including a composite organic interlayer that includes an emissive layer, sandwiched between an anode and cathode. The OLED typically includes a transparent electrode, the anode, disposed between the organic interlayer and a transparent substrate, enabling the emissive layer to provide an illuminated display viewable through the substrate. A metallic cathode is typically arranged behind the organic interlayer and a critical aspect of an OLED is that good contact between the cathode and the organic interlayer is ensured. When light from the environment enters the display device, it travels through the transparent layers and reflects off the metallic cathode. The reflected light distorts the light illuminated by the emissive layer. Therefore, there has been an effort to develop high contrast light emitting devices that absorb rather than reflect environmental light.
Many approaches have been taken to address the problem of distortion as a result of reflection of environmental light. Many of these approaches, however, require an additional apparatus and considerably increased costs. For example, adding a lens outside the OLED has been tried. Another approach has been to form the cathode as a black electrode that includes a layer of carbon. Such an electrode absorbs light and reduces problems that arise from reflection. Adding a layer of carbon to the cathode, however, undesirably creates a cross voltage drop between the cathode and the electron transport layer, typically TAZ, 1,2,4-triazole and causes power levels to rise undesirably. The carbon layer also provides inferior contact between the cathode and the adjacent electron transport layer. To alleviate these problems, a layer of magnesium, Mg, must be added as part of the cathode between the black electrode and the TAZ electron transport layer. The addition of the Mg layer between the carbon and organic layers enhances carrier injection by energy level matching of the Mg and organic materials. The layer of Mg is typically formed using deposition processes that differ from these used to form the organic interlayer. As such, forming a three-layer cathode to include a layer of magnesium, in order to accommodate the cathode further including a layer of carbon is undesirable and costly as it reduces throughput and renders the fabrication process unsuitable for mass production use.
It would be therefore desirable to produce a high contrast organic light emitting device with a light absorbing layer that absorbs environmental light, produces a distortion-free display, is compatible with other materials used to form the device and does not engender an undesirably high power consumption. In particular, it would be desirable to produce such a device that doesn't require a carbon black cathode that is a part of a multiple layered cathode that further includes a magnesium layer.